Genesis and hydrocarbon significance of vesicular welded tuffs: A case study from the Fengcheng Formation, Wu-Xia area, Junggar Basin, NW China

Abstract

On the basis of a large number of core and thin-section observations, the petrologic and pore evolution characteristics of welded tuffs in the Fengcheng Formation in the Wu-Xia area, Junggar Basin, are evaluated, and the genesis of vesicular welded tuffs and the formation mechanism of vesicles are analyzed. Volcanic activity in the Fengcheng Formation is characterized by welded pyroclastic rocks. There are predominant plastic shards with rounded edges and obvious devitrification in the rocks. Plastic debris (fiamme) is commonly deformed and wrinkled, and crystal fragments are usually corroded. A lot of lithophysae occur in the welded tuffs, and the welded texture is obscured due to the occurrence of these lithophysae features. Vesicles are the cavities of lithophysae, which are formed by solidification contraction. They have little or no late filling cement because the volatile constituent and water vapour in the bubbles have escaped with the help of devitrification. Lithophysae distribution within a cooling unit are characterized by vertical zonation: increasing upwards but decreasing gradually later. The cavities of lithophysae have greatly improved the reservoir quality of welded tuffs. Influenced by the intensive welding process, lithophysae are not subject to collapse or damage. In the study area, deeply buried vesicular welded tuffs are important hydrocarbon reservoir rocks, and the vesicular welded tuff has good petroliferous properties. Key words: welded tuffs, vesicle, lithophysa, pyroclastic flow, Fengcheng Formation, Wu-Xia area, Junggar Basin